1. Field of the Invention
The present invention relates to an electromechanical slide-screw tuner system, and more particularly to such a system to be used in load-pull setup for the measurement, characterization and testing of RF or microwave devices. RF/Microwave tuners are electronic devices or mechanical devices which modify in a predictable way the complex impedance seen by the device under test (hereinafter referred to as “DUT”) at a given frequency of operation. The impedance tuner has the capability of generating impedance to the microwave devices close to the conjugate complex of the DUT's internal impedance. This technique of subjecting DUT to variable load impedance or variable source impedance with corresponding load tuner and source tuner, commonly referred to as “load pull”, is used to test transistors for amplifier, oscillator or frequency multiplier applications.
2. Description of Prior Art
The automatic single RF slug slide-screw tuner consists of a housing (1), a coaxial 50 ohm characteristic impedance transmission line (2,72), center conductor (3,73), two slabs as outer conductor ground return (72) and uses a single RF slug (5,70), movable in horizontal and vertical direction into a transmission line. The principle of slide-screw tuners has been first described by LANGE Julius in “Microwave Transistor Characterization Including S-Parameters”, Texas Instruments, Hewlett Packard Application Note 95. The main disadvantages of slide-screw tuner are power limitations and high gamma accuracy limitations when it comes to high gamma. Power limitations are due to corona discharges that can take place between the tuning metallic slug (70) and the central conductor (73) at low impedance, i.e. high gamma. The greater the distance between the metallic slug and the central conductor, the more power can be transmitted, and conversely, the smaller the distance, the less power can be transmitted mainly because of corona discharge. Accuracy limitations also at low impedance are because of mechanical limitations to reproduce accurately a given slug position close to the central conductor, position where sub-micron precision is required.
One enhancement of single slug slide-screw tuner is disclosed in CA Pat. No. 2,311,620 and U.S. Pat. No. 6,674,293 issued to Christos Tsironis on Jan. 6, 2004, which relates to a double slug slide-screw tuner that comprises a housing (51), a first (54) and a second (54′) carriages, said carriages are controlling a first (55) and a second (55′) RF slugs in series in a transmission line (52), said first slug is adapted to act as a pre-matching section and said second slug is adapted to act as a tuning section. This tuner requires an additional sub-micron vertical positioning system for the second slug compared with the central conductor (53) which increases considerably the mechanical construction difficulty and the manufacturing cost. Another shortcoming of this solution is described in the said patent, the technique being in reality two independent tuners connected in series, the combined calibration time may be too long for practical considerations. Consequently, the patent provides alternative methods for calibrating the tuner to cut down the calibration time. But these calibration methods are based on approximations and will therefore give limited accuracy for the measurements.
The dilemma is how to keep the simplicity of operation of the slide-screw tuner and be able to reach high reflection factors, typically gamma above 0.95, required for today's low impedance transistors, without compromising too much on the power level and the accuracy of the measurements.